* changes vmalert query function
for prometheus rules compatibility its better to use labels as map.
it simplifies template evaluation and allow to ignore can't evaluate field error
because map will return default value.
fixes https://github.com/VictoriaMetrics/operator/issues/243
On templates validation stage vmalert does not acutally send queries, so for complex
chained expression validation may fail. To avoid this, we add a blank sample in response
so validation can pass successfully. Later, during the rule execution, stub will be replaced
with real `query` function.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/989
The commit adds a support for template function `query`,
`first` and `value`. The function `query` executes
a MetricsQL query for active alerts. In vmalert we
update templates on every evaluation for active alerts
to keep them up to date. With `query` func it may become
a perf issue since it will fire a query on every execution.
We should keep it in mind for now.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/539
* Initial rules evaluation support.
Rules are now store alerts state in private field `alerts`. Every evaluation updates
the alerts and state. Every unique metric received from datastore represents a unique alert,
uniqueness is guaranteed by hashing ordered labelset.
* merge with master
* cleanup
* support endAt parameter as 3*evaluationInterval for active alerts
* make golint happy
